More TransparentWrapper impls (core::cmp::Reverse and core::num::Saturating) (#269)

* impl<T> TransparentWrapper<T> for core::cmp::Reverse<T>

* impl<T> TransparentWrapper<T> for core::num::Saturating<T>, with a new feature for non-base-MSRV stdlib TransparentWrapper impls.

* Restrict TransparentWrapper<T> for Reverse<T> to cfg(feature = "transparentwrapper_extra").

* nightly_docs for transparent_wrapper_extra

---------

Co-authored-by: Lokathor <zefria@gmail.com>

Cargo.toml

@@ -40,6 +40,11 @@ aarch64_simd = [] # MSRV 1.59.0: support aarch64 simd types
 
 must_cast = [] # MSRV 1.64.0: support the `must` module.
 
+# Adds `TransparentWrapper` impls for stdlib types newer than bytemuck's base MSRV.
+# Current MSRV 1.74.0: `core::num::Saturating`.
+# MSRV may increase if impls are added for newer types.
+transparentwrapper_extra = []
+
 const_zeroed = [] # MSRV 1.75.0: support const `zeroed()`
 
 # Do not use if you can avoid it, because this is **unsound**!!!!

src/transparent.rs

@@ -303,3 +303,20 @@ pub unsafe trait TransparentWrapper<Inner: ?Sized> {
 }
 
 unsafe impl<T> TransparentWrapper<T> for core::num::Wrapping<T> {}
+#[cfg(feature = "transparentwrapper_extra")]
+#[cfg_attr(
+  feature = "nightly_docs",
+  doc(cfg(feature = "transparentwrapper_extra"))
+)]
+unsafe impl<T> TransparentWrapper<T> for core::num::Saturating<T> {}
+
+// Note that `Reverse` existed since Rust 1.19.0, but was only made `#[repr(transparent)]`
+// in Rust 1.52.0 (PR: https://github.com/rust-lang/rust/pull/81879), so we have it under
+// the same feature as `Saturating`, which was stabilized in Rust 1.74.0, so that this
+// impl cannot be used on a version before 1.52.0 where it would be unsound.
+#[cfg(feature = "transparentwrapper_extra")]
+#[cfg_attr(
+  feature = "nightly_docs",
+  doc(cfg(feature = "transparentwrapper_extra"))
+)]
+unsafe impl<T> TransparentWrapper<T> for core::cmp::Reverse<T> {}